Broadband and wide field angle compensator

Abstract

A rotatable compensator configured to transmit non-collimated light over a broad range of wavelengths, including ultraviolet, with a high degree of retardation uniformity across the aperture is presented. In one embodiment, a rotatable compensator includes a stack of four individual plates in optical contact. The two thin plates in the middle of the stack are made from a birefringent material and are arranged to form a compound, zeroth order bi-plate. The remaining two plates are relatively thick and are made from an optically isotropic material. These plates are disposed on either end of the compound, zeroth order bi-plate. The low order plates minimize the sensitivity of retardation across the aperture to non-collimated light. Materials are selected to ensure transmission of ultraviolet light. The optically isotropic end plates minimize coherence effects induced at the optical interfaces of the thin plates.

Description

CROSS REFERENCE TO RELATED APPLICATION[0001]The present application for patent claims priority under 35 U.S.C. §119 from U.S. provisional patent application Ser. No. 61 / 869,065, entitled “Broadband and Wide Field Angle Compensator,” filed Aug. 23, 2013, the subject matter of which is incorporated herein by reference in its entirety.TECHNICAL FIELD[0002]The described embodiments relate to optical metrology systems, and more particularly to systems including rotatable compensator elements.BACKGROUND INFORMATION[0003]Semiconductor devices such as logic and memory devices are typically fabricated by a sequence of processing steps applied to a specimen. The various features and multiple structural levels of the semiconductor devices are formed by these processing steps. For example, lithography among others is one semiconductor fabrication process that involves generating a pattern on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not l...

AI Technical Summary

Benefits of technology

This patent describes a device called a rotatable compensator, which helps to control the phase of light in an optical system. The device includes a stack of four individual plates that are in optical contact. Two of these plates are made from birefringent material, and are designed to form a low order, compound, zeroth order bi-plate. This bi-plate minimizes the sensitivity of retardation across the aperture to non-collimated light. The other two plates are made from optically isotropic material, which helps to minimize coherence effects induced at the optical interface of the plates. In certain embodiments, a single birefringent plate can be used, with a relatively thick plate of isotropic material located on one or both sides to further minimize coherence effects. Overall, this device helps to improve the quality and stability of optical systems.

Problems solved by technology

As devices (e.g., logic and memory devices) move toward smaller nanometer-scale dimensions, characterization becomes more difficult.

Devices incorporating complex three-dimensional geometry and materials with diverse physical properties contribute to characterization difficulty.

Such an ellipsometer, commonly called a rotating-polarizer or rotating-analyzer ellipsometer, is termed “an incomplete” polarimeter, because it is insensitive to the handedness of the circularly polarized component and exhibits poor performance when the light being analyzed is either nearly completely linearly polarized or possesses a depolarized component.

All of these examples employ relatively thick (on the order of one millimeter) waveplates that are suitable for use with highly collimated light, but are not generally suitable for uncollimated light.

All of these designs are sensitive to field angle and are only suitable for use within an ellipsometer employing highly collimated light.

But, these waveplates are typically thinner than the coherence length of the incident light.

As a result, they suffer from phase and transmittance oscillations as a function of wavelength.

This is a particular problem for ellipsometers operating with broadband light.

Anti-reflection coatings reduce the amplitude of these oscillations, but typically only work well in a short wavelength range.

Furthermore, broadband anti-reflection coatings can stress the waveplate, decreasing the retardation uniformity across the clear aperture.

In general, free-standing, ultra-thin zeroth order waveplates cannot withstand the stress of even simple coatings.

But, BK7 does not transmit ultraviolet light, and thus is unsuitable for use within a broadband ellipsometer including ultraviolet light.

This makes them difficult to describe and stabilize.

Furthermore, these compensators require usage in a fixed arrangement with respect to the polarizing element, making them inappropriate for use within a rotating element ellipsometer.

This can be engineered into polymers, but these polymers do not reliably transmit ultraviolet light.

The inventors are not aware of any suitable biaxial crystals.

Existing compensator designs have failed to enable a rotating compensator ellipsometry tool operating with a non-collimated, broadband illumination source that includes ultraviolet wavelengths.

Images